This invention relates generally to MISFET (MOSFET) devices having source and drain regions connected by a gate-controlled channel, and more particularly the invention relates to a MOSFET device having a reduced drain-gate feedback capacitance provided by a recessed shield between the gate and drain electrodes.
The MOSFET device has many electrical applications including use as a RF/microwave amplifier. In such an application, the gate to drain feedback capacitance (Cgd or C.sub.rss) must be minimized in order to maximize RF gain and minimize signal distortion. The gate to drain feedback capacitance is critical since it is effectively multiplied by the voltage gain of the device or C.sub.effective =C.sub.rss (1+gm R.sub.1) where gm is the transconductance and R.sub.1, is the load impedance.
Heretofore, Faraday shields have been employed between the gate and drain electrodes in an attempt to minimize the feedback capacitance. Adler et al. U.S. Pat. No. 5,252,848 discloses a MOSFET structure in which a shield is provided over the gate electrode and which terminates over the drain electrode. The shield comprises a polysilicon layer with resistance of 100 ohms/square or less formed over a nitride film over a stress relief oxide formed directly over the gate. The structure is effective, but the fabrication of the device is complex due to the two polysilicon layers which are required. Weitzel U.S. Pat. No. 5,119,149 discloses a gallium arsenide MESFET structure in which a shield conductor is placed between the gate and drain electrodes without overlapping the gate. The gate to drain capacitance is not minimized since the metal electrode is placed over the passivation dielectric material for the gate structure.
The present invention is directed to a fabrication method and resulting MOSFET device which does not require complex or costly processing and which reduces the gate-drain feedback capacitance without any increase in the input capacitance of the device.